A Fabry-Perot etalon interferometer consists normally of two plane, parallel partially reflecting surfaces formed on a solid glass spacer so that one portion of incident radiation is transmitted directly through while other portions, being reflected between the partially reflecting surfaces before emerging, are transmitted over a longer path.
As described in U.S. Pat. No. 3,824,018, issued July 16, 1974, and assigned to the same assignee as the present application, a Fabry-Perot etalon is adapted to discriminate coherent radiation by making the optical thickness of the glass spacer sufficiently less than the absolute coherence length of the incident radiation (i.e. the absolute of the coherence length of the coherent and incoherent incident radiation) so that the absolute coherent length will be substantially less than the difference between the lengths of the paths of the directly and indirectly transmitted radiation (i.e. the optical path difference or OPD). Then, by changing the path length, by means of tilting the etalon, the intensity of the incoherent radiation transmitted will remain constant, but the coherent radiation modulates. The intensity of radiation transmitted through the etalon is a function of the OPD and of the wavelength of the radiation. The OPD is a function of the index of refraction of the spacer, of the thickness of the spacer and the angle of refraction of the radiation passing through the interior of the spacer. As the etalon is tilted in a scanning mode at a predetermined rate to vary the optical path lengths, and the OPD, the intensity of the transmitted radiation, suitably detected by a photodetector at the back of the etalon, varies in a manner such that the frequency of the radiation as picked up by the detector decreases and reaches a minimum as the etalon swings through a position at which the angle of incidence of the impinging radiation, and hence the angle of refraction is zero. By this means the etalon is utilized to detect the presence and the relative position of the source of the coherent radiation.
Other related patents in the field include U.S. Pat. Nos. 4,170,416 dated Oct. 9, 1979; 4,222,667 dated Sept. 16, 1980; and 4,309,108, dated Jan. 5, 1982. The present invention is intended as an improvement over the forgoing patents for particular installations.